Rigid, one-piece, biaxially stretched shaped body of synthetic resin and method for making the same

ABSTRACT

What is disclosed is a rigid, one-piece, shaped body, made of a biaxially stretched synthetic resin in the form of a sheet at least 1 mm thick, the edge of which does not lie in a single plane, and the surface of which is smaller than a flat initial surface enclosed by the edge and preferably approximates the mathematically minimum surface enclosed by the edge, is suitable according to the invention as a lightweight building element having high resistance to buckling. The new shaped bodies are made by shaping a flat sheet of biaxially stretched synthetic resin at least 1 mm thick, while in the thermoelastic state, so that the edge of the sheet is brought into a configuration which does not lie in one plane, and the surface enclosed by the edge is allowed to shrink back partially while in the thermoelastic state, whereby a shrinkage of the edge of the sheet is prevented by clamping the edge, and the resulting shaped body is cooled below the softening temperature before the edge clamping is loosened.

FIELD OF THE INVENTION

The invention relates to rigid shaped bodies, suitable as lightstructural elements and preferably translucent. Shaped parts of thistype are, for example, transparent domes or barrel vaults made ofacrylic glass sheets. They are primarily used as roof elements. Theaforementioned shaped bodies do not represent the best technicalsolution for all loads encountered in practice. Thus, in an unevenstressing of a transparent dome from above, all the forces involved areconverted into pressure or buckling strains. The resistance to bucklingmay then be exceeded locally and the dome caved-in. This process mayresult in a break in the dome. Simple barrel vaults are even moresensitive in this respect.

DESCRIPTION OF THE PRIOR ART

Transparent domes are extremely rigid structures, which is desirable inmany cases but is often also disadvantageous. For covering cablenetworks, grid shell constructions, and similar substructures which arenot completely rigid, form-retaining essentially rigid shaped bodies aredesired in the construction of larger closed coverings; the shapedbodies should, however, be capable of limited movement without break ifthe substructure is elastically deformed. Barrel vaults in accordancewith DE-OS No. 21 07 728 are essentially more flexible but cannot oftenbe used because of the scanty resistance to buckling mentioned above.

A disadvantage common to transparent domes and barrel vaults is that, inan array of these structural elements to form longer strips or largerareas, the synthetic resin surfaces always are the parts of the roofingthat project highest. With roofings of this type, then, no scaffolding,cables, or the like can be laid on them during repair or maintenancework without endangering the plastic material.

A rigid, one-piece shaped synthetic resin body which can be used as aroof covering element, the edge of which body does not lie in one plane,is known from DE-AS No. 24 30 182. Because it includes a frame which isof one piece therewith, the known body is completely torsion-resistantand is not capable of elastic deformation. Its surface is significantlylarger than the imagined planar surface enclosed by the edge. The shapedbody has the same disadvantages, described above, as other known roofingelements, for example, transparent domes.

SUMMARY OF THE INVENTION

The object of the present invention is to avoid these disadvantages withthe type of shaped body specified in claim 1. It has been found thatthis object was achieved by shaped bodies of a kind that have a surfacesmaller than the imaginary smooth initial surface enclosed by the edgeof the shaped body.

The imaginary flat initial surface is a surface which is generated froma planar surface by warping or bending around one or more fold lines.The edge of the shaped body of the invention always lies in such a flatinitial surface. The surface is geometrically indicated as flat becauseits generatrix is a straight line. Examples of this type of flat initialsurface are cylindrical and conical or saddle-roof surfaces. The edge ofthe shaped body to be sure lies in a flat initial surface of this type,but not in a plane in the narrow sense, that is, not in a plane in whichany number of straight lines can be run through every point thereof.

The flat initial surface enclosed by the edge of the shaped body is notsimultaneously the surface of the shaped body of the invention. Thesurface of the latter differs from the flat initial surface and issmaller. Exceptions aside, the surface of the shaped body of theinvention does not belong geometrically to the class of flat surfaces,since its generatrices are not straight lines. The surface is ananticlastic, i.e. saddle-shaped, warped surface with negative Gaussiandistortion and is not capable of being spread out into a plane. In theideal case, it is a minimum surface, that is the mathematically smallestpossible surface enclosed by the given edge. For reasons which need notbe discussed here, the preferred shaped bodies of the inventionapproximate the mathematically minimum surface, but do not exactly agreetherewith.

By special measures, shaped bodies can be formed which deviate morestrongly from the exact minimum surface, which can be desired because ofstructural as well as design considerations. Such shaped bodies areproduced, for example, by reinforcement of the edge zones or in thesaddle region.

As in the description of the method of making the new shaped bodiesfurther explained below, the form of the shaped bodies of the inventionwhich approximates the minimum surface arises from the flat initialsurface when a biaxially stretched sheet of synthetic resin is permittedpartially to shrink while in the thermoelastic state, with its edgefixed. Because of the shrinking forces latent in the biaxially stretchedinitial sheet, the sheet attempts to shrink back to its original smallersurface area when heated to the thermoelastic state. This shrinkingprocess is limited by the fixed edge. As a result, the sheet shrinks toacquire the smallest possible surface within the given edge, wherebyunder the most favorable circumstances, a minimum surface is almostreached. Prerequisite for the formation of a shape approximating theminimum surface in an only partial shrinkage from the originallybiaxially stretched state. Thus, the shaped body being formed mustcontinue to be biaxially stretched at every point, although to a lesserextent than the originally biaxially stretched sheet which is used.Preferably, the amount of residual stretching is at least still 25%. Themore uniform the distribution of the residual stretching at every pointand in every direction of the shaped body, the closer its shape to themathematically minimum surface.

In a dome or barrel vault surface, the approximated minimum surface hasthe advantage of better resistance to buckling. The surface isoppositely (anticlastically) curved at practically every point in twomutually perpendicular directions. In this way, upon the application ofa punctiform load, a pressure strain arises in one of these directionsand a tensile strain in the other. As a result, buckling will beprevented as long as the tensile strength of the material is notexceeded.

The shaped bodies of the invention are, to be sure, essentially rigidand torsion-resistant, and retain their shape even in the uninstalledstate. However, they are less rigid as domes and can therefore yieldelastically with limited movements of the edge. They are of a singlepiece in the sense that they are not assembled from various separatelymade and detachable parts but consist of continuous, seamless material,which of course does not exclude laminated reinforcing elements possiblymade of other materials. In arranging the shaped bodies to formtransparent strips or extended roof surfaces, the edges in any event arethe construction elements which most project outwardly. In general, theedges are held by stable metal frames capable of supporting maintenancescaffolding and the like. In this way, shaped bodies with base surfacecan be simply made, which bodies can be combined to form large surfaceswithout a break. This suggests the possibility of developing the shapedbodies, with a suitable mounting, as a modular structural system elementand of using them additively for making supported structures of widearea.

Those stretched synthetic resins which have proved their worth in themanufacture of structural elements are suitable materials for the shapedbodies of the invention. The best of these is acrylic glass, because ofits unexcelled resistance to aging and weathering. Polyvinylchloride andpolycarbonate are also suitable, especially when they carry a thin layerof acrylic resin for protection of the surface. To ensure the requiredtorsion resistance, the thickness of the resin should be at least 1 mm.The preferred thickness is somewhere between 2 mm and 6 mm. The area ofa shaped body may be between 1 m² and 10 m². Clear transparent or whitetranslucent materials are preferred, but opaque or transparent coloredmaterials may be used. The original material used for making the shapedbodies is stretched biaxially at least 25%, preferably 40 to 80%, andcan be made from sheet material (which is extruded or polymerized insitu) by stretching while in the thermoelastic state.

Among the advantages of the new shaped bodies is also the possibility ofmanufacturing them in relatively simple fashion without touching thesurface thereof. The procedure is always to bring the edge of the sheetinto a configuration which does not lie in one plane, fix it, and topermit the surface enclosed by the edge to shrink partially while in thethermoelastic state. The shape of the invention then is automaticallyformed under the effect of the liberated restoring forces without theaction of a high or low gas pressure on one side of the plate. Since acompletely contact-free forming of the sheet surface (apart from therestraint at the edge) is possible, damage to the sheet surfaces ispractically excluded. The most expedient method of manufacture isparticularly directed by the desired shape of the shaped body and willbe explained in detail below with the aid of the preferred embodiments.

DESCRIPTION OF THE DRAWINGS

The preferred embodiments are shown in FIGS. 1 to 4, in which theimaginary flat initial surface is designated as a and the shape of thefinished body, approximating the minimum surface, is designated b. Toclarify the shape of the surface, a network of dotted lines with noparticular technical significance is drawn into the imagined flatinitial surface as well as the surface of the shaped body.

FIG. 1 shows the initial surface in the form of a saddle roof;

FIG. 2 shows a rectangular box;

FIG. 3 shows a cylindrical section; and

FIG. 4 shows a diagonally folded square.

The base surfaces of further shaped bodies according to the inventionare shown in FIGS. 5 and 6. The heavily drawn lines are the outlines ofthe base surfaces, the dot-and-dashed lines are the outlines of theoriginal planar synthetic resin sheets used, and the dotted lines arethe fold lines for making the flat initial surfaces. FIGS. 5a and 6bshow possibilities for surface coverings arrays.

For making the shaped bodies of FIGS. 1, 2, and 4, in which the initialsurface has sharp-edged fold lines, a tension frame surrounding the edgeand having flap hinges at the end points of the fold lines is preferablyused. The planar biaxially stretched synthetic resin sheet is clampedinto the frame while cold such that the edge of the sheet is not pulledout of tension by the liberated shrinking or restoring forces whenheated to the thermoelastic state. The frame is then heated, for examplein a hot-air cabinet, until the synthetic resin sheet softens. In thecase of acrylic glass, the thermoelastic state range extends from about130° C. to 160° C. After that, the tension frame is bent around thehinges until the edge assumes the desired shape. The enclosed surfaceshrinks together to a shape approximating the minimum surface in themanner described above. The body is allowed to cool in this shape andcan assume it as a form-stable structure after the tension frame isloosened.

For the manufacture of bodies with curved edges in accordance with FIG.3, a correspondingly curved tension frame is used in which the firstplanar sheet is inserted by bending it elastically. The sheet thenassumes the position shown in FIG. 3a and is fixed in this position. Assoon as the thermoelastic state is reached on heating, the shrinkingbegins and leads to the shape shown in FIG. 3b.

In similar fashion the shaped bodies shown in FIGS. 1, 2, and 4 can alsobe made if a correspondingly bent tension frame without hinges is used.The original planar sheet is heated in a small zone on the fold linebefore clamping, and is bent into the desired shape when softened. Theadjoining, unwarmed surfaces prevent the contraction of the warmedstrips. The sheet is subsequently fixed, warmed to the thermoelasticstate, and finally cooled.

The shaped bodies shown in FIGS. 1 to 3 cover rectangular surface areasand are therefore particularly suitable for setting in a linear orwide-area array. The shaped body shown in FIG. 4 also can be joinedtogether with others to form a trapezoidally corrugated roof surface asshown in FIGS. 4c and 4d.

Shaped bodies having the base surface of a regular hexagon or octagoncan be made for special architectural or esthetic effects if theoriginal sheet is in the form of an equilateral triangle or square andfold lines are made through the center point of adjacent sides. Theresulting hexagonal bodies alone can be closed into a surface, whereasadditional bodies having square bases are required for an array ofoctagonal bodies, as shown in FIGS. 5 and 6. The possibility ofcombining a number of individual minimum surface elements constructivelyto create a complete surface which itself forms a minimum surface shouldnot remain unmentioned.

We claim:
 1. A method for making a rigid one-piece shaped body ofbiaxially stretched synthetic resin in the form of a sheet having athickness of at least 1 millimeter, said body having an edge which doesnot lie in one plane and having a surface smaller than the imagined flatinitial surface enclosed by said edge, which method comprises deforminga sheet of biaxially stretched synthetic resin at least 1 millimeterthick by elastic bending at a temperature below the softening point ofthe resin into a configuration in which the edge of the sheet does notlie in a single plane, rigidly fixing the edge of the sheet, and heatingsaid sheet without touching the surface thereof and with equal gaspressure on both sides of the sheet until said synthetic resin is in thethermoelastic state, whereby the surface of the sheet enclosed by theedge shrinks back partially but shrinkage of the edge of the sheet isnot permitted, and subsequently cooling the sheet below the softeningtemperature of the synthetic resin.
 2. A method as in claim 1 whereinthe edge of said shaped body has two pairwise opposed rectilinear edgesegments and two pairwise opposed arcuate edge segments.
 3. A rigidone-piece shaped body made by the method of claim
 1. 4. A method formaking a rigid one-piece shaped body of biaxially stretched syntheticresin in the form of a sheet having a thickness of at least 1millimeter, said body having an edge which does not lie in one plane andhaving a surface smaller than the imagined flat initial surface enclosedby said edge, which method comprises rigidly fixing the edge of a sheetof biaxially stretched synthetic resin at least 1 millimeter thick,heating said sheet without touching the surface thereof and with equalgas pressure on both sides of the sheet until said synthetic resin is inthe thermoelastic state, and deforming said heated sheet into aconfiguration in which the edge of the sheet does not lie in a singleplane, whereby the surface of the sheet enclosed by the edge shrinksback partially but shrinkage of the edge of the sheet is not permitted,and subsequently cooling the sheet below the softening point of thesynthetic resin.
 5. A method as in claim 4 wherein the edge of saidshaped body consists of a plurality of rectinlinear segments.
 6. A rigidone-piece shaped body made by the method of claim
 4. 7. A method formaking a rigid one-piece shaped body of biaxially stretched syntheticresin in the form of a sheet having a thickness of at least 1millimeter, said body having an edge which does not lie in one plane andhaving a surface smaller than the imagined flat initial surface enclosedby said edge, which method comprises first folding a sheet of saidbiaxially stretched resin along at least one fold line by heating saidresin above its softening point in a small zone along said fold line anddeforming the sheet so that the edge of said sheet is no longer in asingle plane, then rigidly fixing the edge of the deformed sheet andheating said sheet without touching the surface thereof and with equalgas pressure on both sides of said sheet until said synthetic resin isin the thermoelastic state, whereby the surface of the sheet enclosed bythe edge shrinks back partially but shrinkage of the edge of the sheetis not permitted, and subsequently cooling the sheet below the softeningtemperature of the synthetic resin.
 8. A rigid one-piece shaped bodymade by the method of claim 7.